Very fine matter such as clay soil and humus particles are so small they are charged, or at least polar substances. The electrons tend to gather a bit more toward one side of the particles.

Fertilizer and minerals that are ions are attracted to the materials with high CEC, and cling with varying amounts of strength. Ions are charged particles because they have one or more extra electrons or are short one or more electrons. These are often shown with a plus or minus sign such as NH4+.

Plant roots can take those minerals and fertilizers away from the soils. Then more ions can get attached. Thus, 'exchange'.

While the word 'Cation' is used in this concept, in actual fact both positive and negatively charged particles can be part of this system.

Materials with a high cationic exchange capacity make good soils for plants, both in the garden and in the aquarium. The particles need to be small enough for the charge from the electrons to show up, and this is a pretty small particle! However, those individual particles can then be clumped together like most of the ADA product line, and many of the Oil Dri and related products. Good garden soil, too, has a certain amount of clay particles and is clumped together into little bits that we recognize as friable, or 'easy to work' or 'not compacted' and other terms.